Microwave duplexer



p 1958 J. L. CARTER ETAL 2,853,684

MICROWAVE DUPLEXER Filed July 10, 1956 INVENTORS, JOHNL. CARTER LAWRENCE GOULD BY IRVING REINGOLD warm 0mm. muzmomm Oh ,QTTOR/VEY United States PatentO MICROWAVE DUPLEXER John L. Carter, Asbury Park, N. J., Lawrence Gould,

. Boston, Mass., and Irving Reingold, West Deal, N. 1., assignors to the United States of America as represented by'the Secretary of the Army Application July 10, 1956, Serial No. 597,049

4 Claims. (Cl. 333-13) (Granted under Title 35, U. S. Code (1952), see. 266) The invention described herein may be manufactured and used by or for the Government for governmental purposes, without the payment of any royalty thereon.

The invention relates to microwave transmission systems and more particularly to microwave duplexers.

One of the limiting factors in the efficient utilization of higher frequencies in microwave transmission systems is the effective operating bandwidths of the microwave circuit components such as directional couplers and duplexers. At frequencies in the millimeter range, the operating bandwidths of the transmission system are usually limited by the physical dimensions of the duplexer waveguide normally required to isolate the transmitter from the receiver.

It is therefore an object of the present inventionto provide a simple duplexing device capable of operating over a very wide range of frequencies.

It is another object of the present invention to provide a microwave duplexer whose dimensions are not critical for wide range frequency operation. I

In accordance with the present invention there i provided a duplexer especially adapted for use in a microwave transmit-receive system. The duplexer includes a waveguide and discrete polarization selector means coupled to opposite ends thereof. One of the polarization selector means is coupled to the transmitter and the other is coupled to the receiver. The waveguide includes a nonreciprocal polarization rotator for effectively rotating the plane of polarization of the transmitter energy through an angle of 45 and means for reversing the direction of propagation of the rotated plane polarized energy through the rotator whereby the plane of polarization of the transmitter energy is rotated through a total angle of 90. The polarization selectors are relatively oriented such that the receiver is responsive only to microwave energy polarized at a prescribed angle with respect to the transmitter energy greater than 90.

For a better understanding of the invention together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings in which; I

Fig. 1 is a perspective view of the duplexer, partially cut away;

Fig. 2 illustrates the details of the polarization selector means which forms part of the duplexer shown in Fig. l;

4 and Fig. 3 is a section along line 33 of Fig. 1 to illustrate the relative angular positions of the polarization selector means.

Referring now to Fig. l of the drawing, there is shown at a circular waveguide having therein a pre-TR gasfilled section 12 terminated by sealed windows 14 and 16, and a microwave energy polarization rotating device 18 which may comprise a ferrite encompassed by a magnetic field (not shown) as described In Luhrs A Patent No. 2,644,930, issued July 7, 1953, or any other suitable nonreciprocal microwave energy polarization rotator well Patented Sept. 23, 1958 known in the art. Although not essential, it is preferable that pre-TR section 12 and polarization rotator 18 be contiguously positioned along the longitudinal axis of circular waveguide 10. Coupled by conventional means (not shown) to the respective ends of circular waveguide 10 are polarization-selective couplers 20 and 20 which are known in the art as finline couplers and described in June 1955 issue of the Proceedings of the IRE, pages 739-741. Both couplers are identical and the structural details of such a coupler is illustrated in Fig. 2. As shown, each coupler comprises a length of circular waveguide provided with a pair of diametrically opposite thin fins 32 that taper inwardly from the cylindrical wall of waveguide 30 until their opposing edges are separated by a small gap shown at 34. The fins 32 are then curved, brought out through a small hole which provides the input to a rectangular waveguide 36 orthogonally positioned with respect to waveguide 30, and then gradually taper outward longitudinally along the wide walls of the rectangular waveguide. Such a device has the unique property of propagating waves perpendicularly polarized to one another in a manner such that electromagnetic wave energy having a plane of polarization parallel to the fins is coupled only to rectangular waveguide 36, and electromagnetic wave energy having a plane of polarization transverse to the fins is propagated only through circular waveguide 30. For example, if microwave energy is introduced into circular waveguide 30 in the TE mode which has a plane of polarization indicated by the vector E parallel to the plane of the fins, then all of the energy associated with the electrical field E is concentrated from the dominant mode of propagation in circular Waveguide 30 to a finline mode in which the energy is largely confined to the gap 34. This energy is then removed from waveguide 30 by the curved portion of the fins and launched into rectangular waveguide 36 where it is propagated in the plane of polarization indicated by the'vector E parallel to the plane of the fins in rectangular guide 36. If, however, a wave having aplane of polarization indicated by the vector E, (transverse field) is applied to circular waveguide section 39, then this energy will pass on through the circular waveguide relatively undisturbed by the presence of the fins and will emerge as E It has been found that such couplers are capable of operating over bandwidths of at least three to one in frequency.

The respective inputs of coupler 20 and 20', that is, the beginning of the inward taper of the fins in cylindrical waveguides 30 and 30', are coupled to opposite ends of circular waveguide 10. The fins within coupler 20 are oriented with respect to the fins within coupler 20' at an angle which is a prescribed multiple of the angle of rotation provided by polarization rotator 18. With the fins thus positioned, it is apparent that the rectangular waveguides 36 and 36' are oriented in the same relative angular position as their corresponding fins. In the example here illustrated, polarization rotator 18 is adapted to rotate the plane of polarization of the microwave energy applied thereto through an angle of 45 and the fins of coupler 20 are rotated in a clockwise direction as respects the fins of coupler 20' by approximately Thus, the rectangular waveguide 36 will be oriented at an angle of 135 in a clockwise direction with respect to rectangular waveguide 36 as shown in Fig. 3. The free end of coupler waveguide 30 is terminated in a suitable matching load and the free end of coupler waveguide 30' is terminated in a window so that the entire structure may be evacuated in order to withstand high power.

For the purpose of discussing the operation I of the duplexer, it will be assumed that the output of a microwave transmitter is coupled to rectangular waveguide 36 and that it is desired to radiate microwave energy by means of an antenna connected to the free end of circular waveguide 30'. It is also to be assumed that the received echo energy is to be detected in a video receiver coupled to the output of rectangular guide 36. The transmitter energy, which is assumed to be polarized as indicated by vector E is propagated towards polarization rotator 18 by means of polarization-selective coupler 20' as hereinabove explained. As the microwave energy passes through rotator 18, the plane of polarization of the microwave energy is rotated, clockwise in this example, through an angle of 45. After passing through polarization rotator 18, the transmitter energy fires pre-TR section 12 so that, in effect, there is provided a short circuit across circular waveguide whereby the transmitter energy is reflected back through polarization rotator 18 which again shifts the polarization of the energy an additional 45 in the same direction. Thus, the transmitter energy introduced in coupler 20' from left to right is now polarized 90 with respect to the incident transmitter energy and, as a result, the 90 polarized energy will propagate through circular waveguide to the antenna. The received energy is polarized in the same manner as the radiated energy and hence will pass through circular waveguide 30 to polarization rotator 18 without any refiected energy being coupled to the transmitter. As the received energy passes through rotator 18, it is shifted another 45 in the same clockwise direction so that, as the received energy leaves rotator 18, it is shifted a total of 135 with respect to the instant transmitter energy. By this time, pre-TR section 12 has recovered so that it is no longer excited and hence the received energy will pass therethrough to the input of coupler 20. The energy through coupler 20 will now propagate along the fins therein and pass through rectangular waveguide 36 to the receiver with very little attenuation.

With the duplexer arrangement hereinabove described the isolation between the transmitter and receiver was found to be about 80 db. This isolation is due in part to the attenuation through the pre-TR section 12 and also due to the fact that the return echo signal and the power leaking through the pre-TR section 12 are 90 different in polarization.

While there has been described what is at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. In a microwave transmit-receive system, a duplexer coupled between the transmitter and receiver of said system comprising, a waveguide including therein a nonreciprocal microwave energy polarization rotating means adapted to rotate the plane of polarization through a prescribed angle, and a gaseous discharge mean responsive only to the transmitter energy and connected across said waveguide, a first polarization selector coupled between the transmitter and the end of said waveguide adjacent said polarization rotator for propagating the transmitter energy to said gaseous discharge means through said polarization rotator whereby said guide is short circuited to reverse the direction of propagation of the transmitter energy through said rotator, and a second polarization selector coupled between the other end of said waveguide and said receiver, said second polarization selector being oriented with respect to said first polarization selector such that the receiver is responsive only to microwave energy polarized at an angle with re spect to the transmitter energy greater than twice said prescribed angle.

2. The device in accordance with claim 1 wherein said prescribed angle is 45 and .said second polarization selector is oriented at an angle of 135 with respect to said first polarization selector.

3. In a microwave energy transmit-receive system, a duplexer coupled between the transmitter and receiver of said system comprising, a microwave energy transmission circuit including therein a non-reciprocal microwave energy polarization rotating means and a gaseous discharge means adapted to be energized only by the transmitter energy, discrete polarization selector means coupled to opposite ends of said waveguide transmission circuit, the polarization selector means coupled to said transmitter being responsive to both the instant transmitter energy and microwave energy polarized with respect thereto, and the other of said polarization selector means being responsive only to microwave energy polarized at a prescribed angle with respect to said instant transmitter energy greater than 90.

4. In a microwave transmit-receive system, a duplexer coupled between the transmitter and receiver of said system comprising, a waveguide, discrete polarization selector means coupled respectively from the transmitter and receiver to opposite ends of said waveguide, a nonreciprocal microwave energy polarization rotator and a gaseous discharge device longitudinally positioned within said waveguide intermediate said polarization selector means, said polarization rotator and said gaseous discharge device being responsive to the transmitter energy for effectively rotating the plane of polarization of the transmitter energy in a prescribed direction through an angle of 45 and for reversing the direction of propagation of said rotated plane-polarized microwave energy through said rotator whereby the plane of polarization of the transmitter energy is rotated through a total angle of 90 in said prescribed direction, said polarization selectors being oriented relative to each other such that the receiver is responsive only to microwave energy polarized at a prescribed angle with respect to said transmitter energy greater than 90.

Proceedings of the IRE, vol. 43, No. 6, June 1955, pp. 739-741. (Copy in Div. 51.) 

